Absorption Spectra

A hot, dense source like the surface of the sun emits light at every wavelength, a continuous spectrum. But that light has to pass through cooler gas in the sun's outer atmosphere before it reaches you. Electrons in that gas sit on fixed energy rungs, and each one can only jump up a rung if it absorbs a photon with exactly the right energy to span the gap. Photons with that exact energy get soaked up and kicked back out in random directions, so far fewer of them reach your eye along the original path. The result is a thin dark line at that one wavelength, sitting inside an otherwise bright, continuous rainbow.

Absorption and emission spectra are two sides of the same atom. Emission lines show up bright wherever an electron drops down a rung and releases a photon; absorption lines show up dark at the exact same wavelengths, where an electron climbs up. Same energy gaps, same element, same set of wavelengths, just flipped between missing and glowing depending on whether you are looking at the light source directly or through a cooler gas in front of it. The absorbed energy is never lost. The atom holds it briefly as an excited state, then releases it again, either as a re-emitted photon heading off in some other direction, or as kinetic energy handed off in a collision.

This fingerprint lets astronomers read starlight like a label. Every element has its own unique, precisely measured set of absorption wavelengths, so matching the dark lines in a star's spectrum against those known patterns identifies exactly which elements sit in its outer layers, no sample required.